Here's a couple of ideas.
1) Use concept of rational numbers.
Start by assigning 2 variables with a square root, representing each student.
Then multiply the 2 variables together resulting in a whole number. (canceling the square root)
Then say that being by yourself is irrational. But working together is rational.

2) Set up a linear equation to show how the more they work together the higher their grades will be.
For 8th grade a simple one is all that is needed.

where t is hours working together.
Provide a graph or table demonstrating the positive slope.
Good luck